Oliver Schmitz

First-time realization of a stably detached, efficient-particle-exhaust divertor regime in the island divertor at Wendelstein 7-X

Oliver Schmitz

University of Wisconsin-Madison

Friday, September 25, 2020



PSFC Seminars

The island divertor concept is an innovative and promising idea to handle heat and particleexhaust in stellarators. At the recently started stellarator Wendelstein 7-X, this divertor concept plays a central role in the device mission to demonstrate reactor relevant plasma confinement for steady-state time scales of up to 30 minutes in the high-performance campaign phase, OP2, starting in 2022. During the recently concluded first campaign with the island divertor, a large step in the experimental qualification of this divertor concept has been made. In discharges heated with Electron Cylotron Resonance Heating of 5-6 MW, central densities in the range of 0.8-1.2 1020 m-3 have been reached in combination with full divertor heat flux detachment and significant neutral gas compression for the first time. The divertor heat loads drop by an order of magnitude from >5 MW m-2 to below 0.5 MW m-2 with increasing density, and the compression ratio of neutrals in the divertor reaches at least 30 with neutral pressure in the subdivertor volume of >8.0 10-4 mbar. This is compatible with the steady-state particle exhaust requirements for high-performance steady-state operation in OP2. These discharges were held stably detached for up to 30 seconds, which is equivalent to several hundred energy confinement times and beyond the time scales for current relaxation. Stable electron temperatures above 2 keV and ion temperatures above 1.5 keV were obtained during these discharges. No impurity accumulation was seen at constant Zeff~1.5 and the stored energy stayed constant at around 600kJ. This contribution will explain the island divertor concept in W7-X, provide an overview of this recently discovered divertor regime, and describe the status of the physics understanding of these results including modeling of these regimes with the EMC3-EIRENE code.

Acknowledgement: This work was funded by the U.S. Department of Energy under grant DE-SC00014210 and DE-SC00013911 and has been carried out within the framework of the EUROfusion Consortium with funding from the Euratom research and training program 2014-2018 and 2019-2020 under grant agreement No 633053.

The views and opinions expressed herein do not necessarily reflect those of the European Commission.

Bio: Oliver Schmitz is Professor in the Department of Engineering Physics at UW Madison. His research focuses on plasma wall interaction with specific focus on three-dimensional aspects in divertor physics. This includes stellarator divertors as well as tokamak divertors in situation with resonant magnetic perturbation fields or non-axisymmetric plasma facing components. His other research branch focuses on development of high density plasma sources for next generation particle accelerators. He is a member of the AWAKE project at CERN and involved with industry projects to develop new plasma sources for industrial particle accelerators. Prof. Schmitz is the Associate Dean for Research and Graduate Affairs in the College of Engineering at UW Madison and serving on various leadership roles in the community, as co-chair of the National Stellarator Coordinating Committee, Deputy Director of the U.S. Burning Plasma Organization and Secretary and Treasurer of the Division of Plasma Physics of APS.

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